1. Field of the Invention
The invention disclosed and taught herein relate generally to topsides for offshore platforms and related installation methods; and more specifically related installation methods and systems to release quickly the topsides from one or more associated barges temporarily supporting the topsides for the offshore platforms.
2. Description of the Related Art
Offshore platforms provide an infrastructure for drilling, production, or other functions of offshore energy production. The platform includes a lower structure that is at least partially submerged and an upper structure, known as a topsides or deck, above the water level that contains drilling or production equipment, cranes, living quarters, and the like. In shallow water, fixed offshore platforms can be supported by the seabed. In deeper water, floating offshore platforms are typically moored to the seabed due to the difficulty of rigid placement to the sea floor.
One type of a floating offshore platform is a Spar. A Spar is a type of floating oil platform typically used in very deep waters and is among the largest offshore platforms in use. A Spar includes a large cylinder or hull supporting a typical topsides. Due to its size of hundreds of meters in length, the Spar hull is typically floated horizontally to the installation site, and upended in the water, and then the topsides mounted to the hull. The hull does not extend all the way to the seafloor, but instead is moored by a number of mooring lines. Typically, about 90% of the Spar is underwater and is considered a “deep floater.” The hull serves to stabilize the platform in the water, and allows movement to absorb the force of potential high waves, storms or hurricanes. Low motions and a protected center well also provide an excellent configuration for deepwater operations.
Deck or topsides installation is typically a challenge for offshore platforms, particularly for deep draft floaters like the Spar, because the Spar must be upended after transportation to the location site. In the past heavy lifting vessels (“HLV”), including but not limited to, derrick barges have been used for topsides installations on the Spar after upending. Traditionally, the topsides of a floating offshore platform requires multi-lifting, for example five to seven lifts, to install the whole topsides due to the lifting capacity of available HLV. Due to multi-lifting, the steel weight per unity area of the topsides can be higher than that of topsides of any platforms (fixed or floating) installed with a single lifting. If the weight of the topsides is reduced, the weight of the Spar hull may also be reduced.
The same or similar principles are applicable to other offshore platforms to which a topsides can be mounted, whether fixed or floating. The challenge is to mount a large, heavy topsides to the rest of the platform in an offshore or near shore location, where the availability and capacity of lifting vessels may be less than optimum.
One or more barges are generally used to transport a topsides to a floating portion of the offshore platform, such as a Spar hull, for installation thereon. Recently catamaran float-over systems have been used to install the topsides onto the Spar hull. A float-over method is a concept for the installation of the topsides as a single integrated deck onto a Spar hull in which the topsides is first transferred onto at least two barges (called “offloading”) and transported with the barges to the installation site for the Spar hull. At the installation site, the barges are positioned on both sides of the Spar hull with the Spar hull below the topsides, the elevation is adjusted between the topsides and the Spar hull, and the topsides is installed to the Spar hull. Installation of the topsides to the Spar hull by the float-over method can allow a high proportion of the hook-up and pre-commissioning work to be completed onshore prior to load-out, which can significantly reduce both the duration and cost of the offshore commissioning phase. The float-over installation method allows for the installation of the integrated topsides or production deck on a fixed or floating platform structure without any heavy lift operation.
FIG. 1 is an exemplary top schematic view of a topsides loaded on two barges in a catamaran system. In general, a catamaran system 100 includes at least a pair of barges 115a, 115b (generally 115) spaced a distance from each other. A fabricated topsides 110 is removably coupled to the barges 115 through a supporting structure, referenced herein as a grillage system 125a, 125b (generally 125) mounted to the barges 115a, 115b, respectively. The grillage system has attachment points 135 for the topsides on each barge. The number of attachment points can vary depending on the load and size of the topsides and the barges. In general, at least two attachment points are used for each barge, although the number can vary from one to many. In the illustration, barge 115a with grillage system 125a has at least two attachment points 135a′, 135a″ and barge 115b with grillage system 125b has at least two attachment points, 135b′, 135b″. 
Different loads occur on the catamaran system 100 that are not prevalent in a single barge system due to the separation of the barges. During loading and transportation to the desired location, the catamaran system is subjected to several loading conditions primarily due to wave action on the separated barges.
The topsides is typically maintained on the grillage system by gravity. While some operations connect a fork with a locking plate around a guide pin to restrict lateral movement between the topsides and the barges, the vertical movement is not constrained because the fork and locking plate are not welded to the guide pin.
Some efforts have restrained the topsides vertically with additional members welded to other portions of the grillage system besides the guide pin. However, these weldments are removed prior to the complete transfer of the topsides to the hull, and necessarily allow the undesirable vertical movement. Thus, during the transfer, the topsides, partially supported by the hull, can impact the grillage system on the barges by the differential motion between the floating barges and the Spar hull. The impacts can cause system wide shock waves throughout the structures from repetitive impacts on each other, accompanying structural damage, and possible sensitive equipment failure. It is difficult for personnel to cut welds at each weldment in a timely manner to release the restrained topsides from the barges while wave action is causing significant differential movement between the barges and the Spar hull.
Therefore, there remains a need to provide a system with a topsides that can be restrained vertically but released timely upon becoming supported with an offshore platform during an installation procedure.